If the most typical bottom note in Irish traditional music is D and the bottom note is typically a D, how is it that there are "flat sets" that are popular? Are there extra keys added to the bottom range but the fingering otherwise is the same as a D chanter? What are the considerations that go into choosing a set that is D or what would be the advantage for instance of a C sharp set which I see a lot of in the classifieds.

If an article or previous post somewhere explains this topic feel free to refer me to it,

I think the term is transposable instrument. Like whistles , saxophones, written music and fingering will be the same although the pitch differs. Concert D is the bright one often found in pub sessions. Narrow bore D or flat sets being a little mellower often. Pitches other than D can sound great with other instruments but will be useless in many trad Irish music sessions. Not so easy to find musicians able or willing to tune to a flat set ( other than D) but they make great solo instruments.